Chemistry Reference
In-Depth Information
to make soft, short-hold cheese varieties, its non-specific proteolytic action reduces yields
of cheeses whose curd spends a long time in the whey (hard and semi-hard cheese), and
caused bitterness in long-hold cheeses. The heat resistance of the enzyme is also a potential
drawback in cheese plant from which the whey is processed as a food ingredient. The
heat treatment and processing does not eliminate the activity of the coagulant and it can
cause protein breakdown in other food products in which whey protein is a supplementary
ingredient (sausages, meat pies, soups, etc.).
To overcome these problems, dairy enzyme producers have developed heat-labile versions
of the
R. miehei
coagulant ('TL' and 'XL') using chemical oxidation to modify methionine
side chains. These enzymes can be denatured by pasteurizing the whey and they are also
less specific than the native proteinase. These coagulants are a good alternative to microbial
chymosin (see below) in the manufacturing of 'vegetarian' cheeses, but the texture of hard
cheese made with them becomes crumbly more quickly than that of cheese made with
chymosin. Also the flavour profile of hard cheese made with fungal rennet is not the same as
chymosin-made cheese.
The most widely used alternative to calf rennet in the cheese industry worldwide is
fermentation-produced chymosin (FPC), which currently accounts for half of the world
production of enzyme-coagulated cheese.
7
It is produced by large-scale fermentation of GM
Kluyveromyces lactis
or
Aspergillus
niger
. In both cases, the microorganism has been modified using gene technology by the
incorporation of the calf prochymosin gene into the host organism with a suitable promoter
to ensure its efficient secretion into the growth medium. The enzyme is relatively easy to
harvest and purify from the culture, unlike the earlier production system using
Escherichia
coli
to produce chymosin in inclusion bodies.
1, 8
Recently, a new FPC from camels (
Camelus dromedarius
) has become available,
7
and its
suitability for Cheddar cheese is documented by
9
showing that the enzyme's high C/P ratio
resulted in yield improvements even over calf-derived FPC.
The details of cheese making with alternative rennets and coagulants are beyond the scope
of this chapter and the reader should study
Technology of Cheesemaking
10
for full coverage
of the topic.
5.2.3 Production of rennets and coagulants
Animal rennets are secreted from the stomach mucosa as inactive proenzymes that can
easily be extracted by maceration with water, weak brine or a buffer solution. A preservative
(usually sodium benzoate) is normally added at this stage to prevent microbial growth during
the next stages of production, involving filtration and acidification to activate the proenzymes.
After neutralization to pH 5.5 and a second filtration to clarify the extract, the preparation is
standardized to the 'advertised' milk-clotting activity, sterile filtered and packaged as a liquid
enzyme product to be transported and stored refrigerated. Animal rennets are not purified
products, but contain whatever enzymes were secreted by the mucosal tissue at the time of
slaughter. However, the enzymes in good-quality calf abomasal tissue are mainly chymosin
and pepsin and only standardization is necessary.
Microbial coagulants are produced by submerged fed-batch fermentation of the production
organisms
Rhizomucor
or
Cryphonectria
. The fermentation is usually for several days,
after which the enzyme is recovered as a crude filtrate, concentrated by ultrafiltration and
standardized. No attempt is made by the manufacturer to purify the product by removing other
co-produced enzymes such as lipases and starch hydrolases, though the production strains of
